The present invention relates generally to improved methods and formulations for use in preparing thermally conductive dielectric mounts for printed circuits including substrates used in their production, and including dielectric layers for improving heat transfer from the circuit elements and from heat generating semi-conductor devices. Thermal energy generated during operation of these devices must be dissipated through transfer to a heat spreader or dissipater such as a heat sink or heat spreader. More specifically, the present invention relates to techniques employed for preparation of mounting pads for circuitry and components, the pads being highly thermally conductive and having exceptional dielectric properties. Those properties are combined with exceptional mechanical properties as well. In particular, the techniques of the present invention facilitate the fabrication of a substrate to serve as a mounting surface for circuits and semi-conductor devices, wherein there is a substantial reduction in the thermal impedance normally associated with the interface created between dissimilar layers. The processes and formulations of the present invention utilize thermoplastics having high glass transition temperatures, such as those preferably in the range of 170xc2x0 C. and higher, thus serving to preserve the exceptional thermal, electrical, and mechanical properties when operating at elevated temperatures.
Thermal properties of substrates and/or mounting pads are typically measured in and through the bulk material. As device and system technology has evolved, efforts have been focused upon the utilization of thin interface surfaces, thus enhancing the rate of heat transfer through the substrate. Because of the improved and enhanced capability of semi-conductor devices, and given the performance limitations imposed by the dielectric properties of the substrate material, a lower limit on substrate thickness has typically been recognized. Due to the typical lack of uniformity in the thermal properties of the polymer layer or component, such as introduction of pinholes and voids, a practical design parameter will include a significant safety factor to preserve the dielectric characteristics. Another feature recognized with the utilization of thin polymer films is the importance of considering the significance of interfacial and/or contact resistance. In other words, in the interests of decreasing the overall thermal impedance of a mounting system, as the thickness of the polymeric component decreases, the importance and/or significance of contact resistance correspondingly increases. It has been found that in accordance with the present invention, interfacial contact resistance is lowered substantially, while at the same time uniformity of the polymeric layer is significantly increased.
The dielectric layers of the present invention are typically interposed between highly adherent metallic layers. The steps in the process include the creation of an enhanced bond between each of the metallic foil layers and the centrally disposed thermoplastic layer, with the processing steps further assuring a formation and/or creation of a thermoplastic dielectric layer which is highly integral and free of pinholes or voids and which has low interfacial or contact resistance at the foil/polymer bond. In other words, one highly advantageous feature of the present invention is the creation of a bond between the metallic foil layer and the polymeric dielectric layer which has highly desirable thermal conducting characteristics. In particular, this bond has demonstrated a significant reduction in impedance, it being believed that this reduction is due to the creation of surface-to-surface foil/polymer bond with low contact resistance, with the contact resistance at such interfaces otherwise being responsible for an increase in overall thermal resistance across a multi-layer assembly, such as those present in many multi-layer arrangements.
In the past, various techniques have been employed for the incorporation of thermoplastic polymeric dielectric layers between opposed metallic foils. In most typical applications, the dielectric and/or polymeric core has consisted of epoxies, bismaleimides, polyesters such as polyethylene terephthalate and polyethylene napthalate as well as a certain phenolics and the like. While such materials have provided certain advantages and benefits, they were not without accompanying disadvantages, such as, for example, low thermal and moisture stability along with certain other relatively poor mechanical properties. Still other disadvantages included lack of integrity in the polymeric layer occasioned by the formation of pinholes and/or voids, as well as a relatively high interfacial thermal contact resistance resulting in greater thermal impedance across the interfaces and the assembly. It is now recognized that any lack of uniformity in surface wetting between the metallic foil and polymeric dielectric layers can dramatically reduce the performance and effectiveness of the assembly, including the semi-conductor devices and associated circuitry due to a relatively high thermal impedance and/or a loss or compromise in the dielectric properties. Poor performance in either property may result in the production of unacceptable systems, subsystems and/or components.
The present invention directed primarily to the preparation of thermoplastic metal-clad laminates for use in circuitry which includes heat generating semi-conductor components. The metallic foil component typically comprises copper or aluminum, with the thermoplastic comprising a polymer composition with a high glass transition temperature selected from the group consisting of polysulfone (PS), poly-ethersulfone (PES), poly-phenylsulfone (PPSU) and poly-etherimides (PI). Each of these thermoplastic materials is commercially available and used in the production of high performance engineering plastics. In addition to their highly desirable thermal and electrical properties, these thermoplastics, when treated in accordance with the present invention, exhibit high chemical resistance and a relatively low modulus, particularly when compared with typically employed thermosetting polymers. The thermoplastic polymers utilized in accordance with the present invention are preferably blended or filled with thermally conductive particulate, such as, for example, alumina, boron nitride, or mixtures thereof. Other fillers are also suitable, including aluminum nitride, silicon carbide, silicon nitride and the like.
In accordance with the present invention, a printed circuit board is prepared pursuant to the following steps or process operations. Initially, a performance thermoplastic-based resin dispersion is prepared, with the dispersion preferably comprising between about 10-25% resin balance solvent or carrier. Suitable solvents or carriers include those typical of the flex circuit industry where thermoplastics are utilized extensively, such as gamma butyrolactone. Thereafter, the dispersion is applied as a coating onto the surfaces of each of a pair of metallic substrates, with the coatings thereafter being dried. Upon release of the solvent or carrier, a moderately adherent film is formed on the surface of the substrate. The coated substrates are then clamped in face-to-face contact while under pressure, and exposed to a temperature of between about 200xc2x0 C. and 350xc2x0 C. for a period of greater than about 30 minutes. During the heating cycle, the coated metallic substrates continue to be subjected to a force sufficient to create unit pressures of between about 100 psi and 800 psi. The thermal exposure time taken together with the application of force results in the creation of a metal/plastic/metal laminate with not only exceptional thermal and electrical properties, but also highly desirable mechanical properties, including a relatively low modulus and high chemical resistance. The selection and utilization of a pair of coated metallic substrates which are bonded together under heat and pressure results in the creation of a highly uniform polymeric layer uniformly bonded at its opposed surfaces to metallic foil. The interfacial bond is both homogenous and uniform.
Therefore, it is a primary object of the present invention to provide improved printed circuit substrates utilizing a thermoplastic polymer having a high glass transition temperature, with the polymer exhibiting exceptional thermal, electrical and mechanical properties.
It is a further object of the present invention to provide an improved printed circuit board comprising a metal/plastic/metal laminate wherein the plastic component is a thermoplastic polymer selected from the group consisting of polysulfone (PS), poly-ethersulfone (PES), poly-phenylsulfone (PPSU), and poly-etherimides (PI). It is a still further object of the present invention to provide an improved polymeric core for a metal/plastic/metal printed circuit board wherein the plastic component is a thermoplastic polymer having a high glass transition temperature, and with the polymer being loaded with a thermally conductive particulate filler such as alumina, boron nitride, aluminum nitride, silicon carbide, silicon nitride or mixtures thereof.